MongoDB Find and Join Collections

MongoDB is a document-oriented database program, which means that it stores data in JSON-like documents that can have varied structures. MongoDB can be easily used with Node.js and other programming languages.

In MongoDB, it is possible to join collections using the aggregation framework. The aggregation framework is a MongoDB command-line tool used to process data inside the database. In this tutorial, you will learn how to use the MongoDB aggregation framework to join two collections.

Step 1: Create the collections

Before we can join collections, we need to create them first. To do that, we will use the following commands in the MongoDB shell:

use test_database
db.createCollection("users")
db.createCollection("posts")

Step 2: Insert some data

After creating the collections, we need to insert some data into them. The data inserted in the collections will be used in the join operation. We will be adding two users and some posts associated with those users.

db.users.insertOne({"_id": 1, "name": "John Doe"})
db.users.insertOne({"_id": 2, "name": "Jane Doe"})
db.posts.insertOne({"_id": 1, "title": "Post 1", "user_id": 1})
db.posts.insertOne({"_id": 2, "title": "Post 2", "user_id": 2})

These commands will create two users with unique IDs and some posts associated with them.

Step 3: Join collections

Now that we have created the collections and added data, we can start joining them. To join the collections, we will use the $lookup operator in the aggregation framework. The $lookup operator is a pipeline stage used to perform a left outer join between two collections.

db.users.aggregate([
  {
    $lookup:
      {
        from: "posts",
        localField: "_id",
        foreignField: "user_id",
        as: "posts"
      }
  }
])

In the above example, we are joining the users and posts collection, and we are getting all the posts associated with each user. The from field specifies the collection to join with, localField is the field in the users collection, and foreignField is the corresponding field in the posts collection. Finally, as is the name of the new array field that contains the joined documents.

Conclusion

MongoDB is a powerful NoSQL database that supports joining collections using the aggregation framework. With $lookup, we can easily join collections and perform complex queries on our data. In this tutorial, you learned how to create collections, insert data, and join them using the $lookup operator.